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Old 23rd Dec 2011, 4:14 pm   #1
Don_Zalmrol
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Default Stereo valve amp not working.

Hello,

My stereo valve amplifier is acting strangly,
This amplifier was a self-build kit from a store in Holland.

http://jukebox-revival.eu/index.php/...er-kits/id/23/

Like the ones you see in the link, it was the previous version of the EL84/12AX7 Amplifier, mine is the VT96PP kit.

Now ontopic:

This morning my amplifier acted strangely, the 2nd valve is getting very very glowing red hot, even the markings from JJ are faded out and the sound screeching…

The rest of the valves seem to be operating normal…

I've opened the bottom to check the circuit and everything seems to be normal.
None of the resistors or pcb seems to be burned or damaged.

Is it just the valve that is worn out or something else?

I will try this evening with switching them and look how they act.

Thank you in advance!
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Old 23rd Dec 2011, 5:36 pm   #2
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Default Re: Stereo valve amp not working.

Assuming it's one of the EL84 valves that is showing signs of distress, something has changed such that the distressed valve is passing too much current. In vintage kit, the coupling capacitor to the grid of the EL84 would be the immediate suspect; this is still feasible, but much less likely with modern components.

Other suspects are the cathode bais resistor and its associated decoupling electrolytic capacitor.

Are you able to take voltage measurements on the offending valve and post them here? Voltages of interest are on g1, g2, cathode and anode of the EL84. Data on the EL84 can be found here http://tdsl.duncanamps.com/show.php?des=EL84

A faulty valve is possible, but JJ are one of the more reputable manufacturers so this would not be my first suspect.

Don't run the amp for any length of time with this fault as damage to the output transformer could occur. The valve that is running hot may be OK but will also be damaged if run like this for any length of time.

John
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Old 23rd Dec 2011, 6:03 pm   #3
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Default Re: Stereo valve amp not working.

Thank you for your reply jjl

Sorry, the kit uses 4x E34L valves, 2 ECC82 and 1 ECC83S.
The problem is with one of the E34L valves, I will check the voltages on the pins.

Where do I measure the voltages on this one?

http://tdsl.duncanamps.com/show.php?des=E34L

Last edited by Don_Zalmrol; 23rd Dec 2011 at 6:22 pm.
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Old 23rd Dec 2011, 6:29 pm   #4
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Default Re: Stereo valve amp not working.

It sounds like the biasing of the valve is out of specification. Check all components around the valve, as has already been stated. In particular cathode resistor/by-pass electrolytic capacitor(if fitted), control grid coupling resistor(s) in particular the biasing resistor (or the pre-set variable resistor if the bias is adjustable) Also the screen grid resistor. Connect a voltmeter across the cathode resistor and tell us what the value of the resistor is and what voltage you have across it.
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Old 23rd Dec 2011, 7:01 pm   #5
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Default Re: Stereo valve amp not working.

As you can see in the attachments, this is my amplifier.
When you look inside the PCB looks to be in good shape. I've also looked at the top plate of the PCB (where the valves reside) and the 4 resistors and 2 capacitors are also good.

I will measure the voltage between the points you've asked. But on what pin do I measure the cathode resistor?

Sorry, I'm not a pro on valve circuits…
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Old 23rd Dec 2011, 7:52 pm   #6
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Default Re: Stereo valve amp not working.

We need to see the voltages on pins


3 Anode
4 Screen grid (g2)
5 Control grid (g1)
8 Cathode

The TDSL page shows the layout of the valve pins from beneath, so they match the points where you need to take measurements on the PCB.

John
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Old 23rd Dec 2011, 7:53 pm   #7
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Default Re: Stereo valve amp not working.

Have you got a circuit diagram for this amp? If so can you post it so that techs can help you more easily. The E34L is supposed to be an up-rated EL34, but the pin outs are the same. Pin 8 is the cathode, pin 5 control grid, pin 4 screen grid, pin 3 anode. High voltages exist in this circuit and if you are not confident of your ability please seek assistance from someone locally who is, it can be dangerous. You need to measure the voltage across the cathode resistor while it is working with no signal input. Having studied your pictures, I can see 4 pre-set variable resistors, these are likely to be the bias adjustment pots, and its likely that a component associated with the one supplying the over-run valve is faulty, or the variable resistor itself. Make sure the cathode resistor is within tolerance, and measure its value with the amp switched OFF, measure the voltage across the resistor when the amp is switched ON, then using Ohms law V=IR I(amps)=V/R calculate the current through the resistor. As a rough guide set the variable resistor connected to the valve so that the current is approx 360ma. (0.36A).
Once you have identified the faulty component and repaired it, set all the valves to run at the same bias, and check that all the grey anode plates are not glowing, if they are, reduce the bias.
Bill
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Old 23rd Dec 2011, 8:59 pm   #8
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Default Re: Stereo valve amp not working.

In the attachments are the diagrams of the amplifier.

Do I need to measure the voltages between the pins described above and the GND or between 3&4, 5&8?

I'm going to measure the voltages and report it back to you guys.

But if I find it too complicated or afraid of wrecking it further, I will stop and go back to the store in the Netherlands and let the retailer take a look at it.

The valve circuit is missing 2 connectors and GND sign on the left-side of the selector.
My scanner won't accept A3 paper…
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Last edited by Don_Zalmrol; 23rd Dec 2011 at 9:01 pm. Reason: Circuit note
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Old 23rd Dec 2011, 9:08 pm   #9
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Default Re: Stereo valve amp not working.

All readings are taken with respect to chassis unless stated otherwise. Connect the negative (black) lead of you meter to chassis and touch the appropriate tags on the valve holder with a probe attached to the red lead. Be careful not to touch more than one tag at once, or short a tag to chassis.

I note that this amp has a separate bias supply with an adjustment pot for each valve. It's possible that one of these pots is maladjusted or has a dirty track. Did you set up the bias voltages when you built the amp?
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Old 23rd Dec 2011, 9:24 pm   #10
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Default Re: Stereo valve amp not working.

Ok, thx!

3# 502VDC (Anode)
4# -38VDC (Screen)
5# 503VDC (Control Grid)
8# 0V Cathode.

This was tested without a valve plugged into the system.

I've also switched valve 1 and 2, and the right side anode burned a little bit hotter than the other one.

I've tested the voltages on valve socket 1 and these are around the same like socket 2, with a +- 1-2V discrepancy.

Last edited by Don_Zalmrol; 23rd Dec 2011 at 9:28 pm. Reason: Valve socket 1 tested
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Old 23rd Dec 2011, 9:36 pm   #11
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Default Re: Stereo valve amp not working.

Here is a picture of the two valves switched from sockets.
If you look closely you can notice that the second valve "burns" hotter then valve #1 and that is more plasma visible then also in valve #1.

The picture was taken in front of the two valves.
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Old 23rd Dec 2011, 9:39 pm   #12
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Default Re: Stereo valve amp not working.

I think you've counted the valve pins the wrong way. From under the chassis they number clockwise from the gap in the pins or the spigot. The -38V suggests this is the control grid.

You need to take readings with all the valves plugged in, as the HT will rise if less current is drawn.
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Old 23rd Dec 2011, 9:45 pm   #13
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Default Re: Stereo valve amp not working.

Ah, okay.

Sorry, I will take the new readings.
But when I plug in #2 valve in the #2 socket the anode shield gets very bright red.

So I will switch them from place for the measurements.

And I did not set up the BIAS, but the retailer from Jukebox-Revival in the Netherlands.
The amp was running fine for more then 2 years.
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Old 23rd Dec 2011, 9:57 pm   #14
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Default Re: Stereo valve amp not working.

3# 462VDC and stable (Anode)
4# 465VDC and stable (Screen)
5# -38VDC and stable (Control Grid)
8# 0VDC

And yes the valve is plugged in and I'm measuring from the back of the PCB with the pin layouts next to in mirror image
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Old 23rd Dec 2011, 10:05 pm   #15
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Default Re: Stereo valve amp not working.

The cathode resistor is a low value (10R), but I'd still expect to see some volts on the cathode pin 8. Do the other output valves have a voltage on pin 8?

With valves in, how do pins 3, 4, 5 and 8 of another output valveholder compare?
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Old 23rd Dec 2011, 10:17 pm   #16
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Default Re: Stereo valve amp not working.

One thing I'm not clear about. Is it the same valve which always gets hot regardless of which holder it's plugged into, or is the fault always associated with the same valveholder?
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Old 23rd Dec 2011, 10:21 pm   #17
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Default Re: Stereo valve amp not working.

I just tested the other valves and all have on pin 8, 0V. When I look closely and measure them, I notice that pins 1, 2, 3, 4, 6, 7 & 8 are connected to the GND

I compared the voltages on the other pins and all have the same output with minor discrepancies of +-1 to max 5V.

It is almost like there is no fault in the circuit, but when I put valve #2 back in valve socket 2 the "overheating" occurs again.

Is it possible that valve #2 is "almost used up" by maybe a bit of to much BIAS power from socket 2?

Because when I put valve 1, 3 or 4 in socket 2 the light up a bit brighter than in their original sockets…
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Old 23rd Dec 2011, 10:22 pm   #18
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Default Re: Stereo valve amp not working.

And on every socket pin 8 shows 0V
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Old 23rd Dec 2011, 10:25 pm   #19
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Default Re: Stereo valve amp not working.

It's impossible that all pins except pin 9 are connected to ground. You'd never get any voltages if that were the case.

You may need to switch to a low voltage range to get a reading on pin 8.

I would say that you have one faulty output valve which you should replace.

Having replaced the valve you'll need to reset the bias as described in the instructions. This may mean adjusting the bias pot to get the correct negative reading on pin 5, or you may have to measure the anode current and adjust it to a specified value.

I would make sure that pin 5 of the valveholder is atually touching pin 5 of the valve.
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Old 23rd Dec 2011, 10:29 pm   #20
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Default Re: Stereo valve amp not working.

Ah, ok. Thank you.
I will order a replacement valve.

Do I need to re-adjust the BIAS power for the new valve?

About all the pins, that is an error of me.

And something I am curious about is why isn't valve #2 not overheating on the other valve sockets?

Thank you for your help and happy holidays!
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